> Doug et al.,
>
> We use a sinusoidally modulated HeNe. That leads to interesting questions such as: What's the better place on the sine wave to mark - the peak or the zero-crossing.
>
> Other than the somewhat touchy alignment of the focusing lens on the return beam, the experiment is straightforward, with good results. And it gets the students out of the confines of the lab for a change of pace.
>
> (Bonus benefit. Since we can use a few mWs of power, our experimental path can use the basement corridor that includes parts of both the Physics and Chemistry buildings, making it an inter-disciplinary experiment ;>)

We also share a hallway with Chemistry!,
for this experiment, about 100 m (x2 round trip, if they use a bounce mirror).

It is fun with a passively Q-switched laser, with < 10 ns pulses, like one of these;
(it makes a good, bright, 532 nm pulse out).

http://www.crystalaser.com/new/q.html

We have used spinning mirrors and gating with galvo-choppers with CW lasers,
(but the "pulses" do end up sine-shaped, which may be good pedagogically, as Terence notes).
also have added a fixed slit mask, down range,
(that can help form or gate the pulse, at the cost of intensity).

A 100 MHz 'scope is the minimum, imho, much easier with a 300 MHz.
It is also cool to do with an OTDR, using media other than air (determine N via time delay).